As an alternative solution:
Manual and API reference.
You'd have to create a custom shader for the one side, but it would be pretty easy to get the default shader being used for the left (there's a download for the default shaders here) and modifying it so that the color/brightness/lighting is altered so that it has the effect you desire.
Replacement shaders operate on the camera level, so the entire scene is rendered a second time (as is normal) but with the override shader being used in place of the object's default shader (with some simple rules involving shader tags). Your shader will probably need two subshaders, one with a tag that renders objects in the original manner, the other that renders them darker. It'll take a little tweaking and experimentation on your part, but it's not hard.
material.SetOverrideTag() may come in handy.
I use it to achieve a wall-hack effect in a game I'm working on. The two cameras are placed at the same point (as I'm not dealing with VR) with the second camera (the one that has the replacement shader set) rendering second and on top of the previous camera.
My replacement shader only renders the edges of the model meshes, rather than faces, but if you set yours to use a different lighting model and darken the result, you'll achieve the effect you desire. Both screenshots were achieved during a single program execution by manipulating the object inspector to swap override tags (this may come in handy if you wish to have a control that allows the user to turn the diplopia effect on and off).